The invention relates to a braking control system for automotive vehicles and, in particular, a hydraulic control system. The invention also relates to mixed braking systems as provided in hybrid vehicles (vehicles having an electric drive and an internal combustion engine drive) comprising a hydraulically actuated braking system and an electric braking system using the electric traction motor(s) as electric generators.
In the known prior art of hydraulic braking systems of automotive vehicles, a brake booster essentially has a housing comprising two chambers (the front chamber or vacuum chamber and the rear chamber or working chamber) separated by a mobile diaphragm fixed to a piston. A control rod may be displaced to the front of the vehicle when the driver of the vehicle actuates the brake pedal. This displacement of the control rod is transmitted to a plunger which actuates assistance means and the piston of the servomotor. Generally, said assistance means comprise a three-way valve, the actuation thereof making it possible to interrupt the communication between the vacuum chamber and the working chamber of the servomotor and to open said latter chamber to the ambient atmosphere. Given that the vacuum chamber is normally under a vacuum and due to the difference in pressure between the two chambers, a boost force is exerted on the piston which separates the two chambers. The piston is, therefore, displaced to the front, acting on a push rod used to actuate the master cylinder of the braking circuit.
The control rod which is actuated by the braking control member (brake pedal) is in contact with the piston of the servomotor, which is in contact with the push rod which acts on the piston of the master cylinder. The different elements which couple the brake pedal to the piston of the master cylinder are thus in contact with one another. The driver thus senses the reactions of the braking circuit via the brake pedal.
However, if a device of the vehicle displaces brake fluid from any point of the braking circuit to the master cylinder, a reaction will be produced in the region of the brake pedal and this reaction will be felt by the driver. For example, during a braking procedure which has the effect of locking the wheels of the vehicle, the anti-lock braking system (ABS) has the function of reducing the braking and thus removing brake fluid from the wheel brakes to inject it into the master cylinder. Alternatively, in electronic stability programs (ESP), a hydraulic unit is capable of acting on one or more braking circuits independently of the control of the brakes and this action is also felt in the region of the brake pedal and if the driver brakes during this operation of the ESP, he/she will feel a variation in the braking sensation which does not necessarily correspond to the braking sensation which the driver normally feels.
It is also appropriate to mention that the injection of brake fluid into the braking circuit via the hydraulic unit may cause the backward movement of the pistons of the master cylinder and this also has the effect of causing a backward movement of the brake pedal. If this occurs when the driver exerts a relatively high braking force, the backward movement of the brake pedal is absorbed by the ankle of the driver which at the very least may be unpleasant for the driver. This may even be the cause of physical injury during an accident, in particular a front impact, occurring when the driver exerts a high pressure on the brake pedal.
It may, therefore, be useful to dampen, or even to suppress or to absorb the retransmission of all these forces which are produced in the braking circuits and/or in the master cylinder in the direction of the brake pedal.
One solution to resolve these drawbacks is to provide an electric control of the control system and to provide a system which applies mechanical commands to the brake pedal which simulate braking sensations which the driver usually feels with a conventional braking system in the same braking conditions. In the remainder of the description, said device is referred to as the “brake-feel simulator”. In such a system, the real braking devices are thus disconnected from the brake-feel simulator which, when braking is reapplied, applies forces to the brake pedal.
However, such systems are costly relative to conventional hydraulic braking systems. The introduction of electric controls and connections generally poses problems of reliability. These systems are thus all the more costly if the same demands of reliability and safety are imposed thereon as in conventional hydraulic systems.
Moreover, so-called hybrid vehicles which have both at least one electric traction motor powered by batteries for the propulsion of the vehicle and an internal combustion engine (using petrol, diesel, gas or any other fuel) are generally provided with electric brakes in which the braking is obtained by regenerative braking and energy recovery from the electric traction motor of the vehicle. The electric traction motor thus functions as an electric generator, the recovered electrical energy being used to recharge the batteries which is advantageous for the use of the vehicle.
In these systems it is also possible to vary the electric braking torque. During braking, the electric brake is not always applied to a maximum. This may be the case, for example, with the use of radar providing information about road conditions or during a more or less rapid maneuver of the brake pedal (for example, hard pressure followed by a release of the brake pedal).
It may also be provided to transmit an inverse current to the electric motor, either for safety reasons or for reasons of the feel of the brake pedal.
However, electric braking is not entirely satisfactory due, in particular, to the gradual increase in its efficiency and the reduction in efficiency at low speeds. It is appropriate, therefore, to provide the vehicle with a braking system which is able to remedy the drawbacks of the electric braking system.
Hydraulic control systems using brake boosters have proved advantageous and it is known to provide a hydraulic braking system which might provide complementary braking to the electric braking system. The hydraulic braking system is thus not directly coupled to the brake pedal but is controlled via an intermediate control device.